This invention relates to the fabrication of metallic sandwich and integrally stiffened structures formed particularly from aluminum or its alloys, and is particularly directed to a method of making such structures by superplastic forming and diffusion bonding (SPF/DB), employing improved procedure for removing surface oxides of the aluminum or aluminum alloy metal blanks in selective areas to permit diffusion bonding of the resulting clean aluminum or aluminum oxide blanks in such selected areas.
Structures have been successfully produced from a number of titanium-based alloys by the Superplastic Forming/Diffusion Bonding (SPF/DB) process. Such structures are producible because many titanium alloys exhibit the two essential physical properties required for SPF/DB, namely ability to be diffusion bonded and superplasticity. Other alloy systems, such as aluminum and aluminum-based alloys, also have these properties, but have not been successfully applied to the SPF/DB process because of the difficulty in eliminating surface oxides, which inhibits diffusion, thereby preventing successful diffusion bonding. Titanium is unique in that it can dissolve its own oxide, within the SPF/DB operating temperature, and therefore allows diffusion to proceed uninhibited, whereas there is only limited solubility of oxygen in metals such as aluminum and its alloys.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause comingling of atoms at the joint interface.
After the bonds between adjacent metal blanks are formed during diffusion bonding, inert gas pressure, such as argon or helium, is applied to the interior network to superplastically form the unbonded portions of the adjacent metal sheets.
U.S. Pat. Nos. 3,927,817 and 4,220,276 are illustrative of processes for fabricating superplastically formed/diffusion bonded structures, employing metal blanks of a titanium alloy.
U.S. Pat. No. 3,500,532 discloses employing an arc in a reducing atmosphere, for the purpose of cleaning metal surface areas of various metals, followed by cold welding the metal surfaces together.
It is known to clean metals, including aluminum with mercury to remove oxide, prior to welding, as disclosed in U.S. Pat. No. 1,196,118.
Rockwell International Report: "Interdisciplinary Program for Quantitative Flaw Definition Special Report Second Year Effort," for period of July 1, 1975 to June 30, 1976, Contract No. F33615-74-C5180, Page 100, discloses fabrication of ultrasonic test standards by a process including removal of the oxide from the entire surface of aluminum sheets D.C. sputtering the surface of the aluminum prior to bonding in an ultra high vacuum system. However, there is no disclosure in the article as to the details of the D.C. sputtering procedure employed therein. The clean surfaces of such aluminum specimens are brought into contact without removing these aluminum specimens from the vacuum, the bondline heated and a force applied sufficient to bring about diffusion bonding of such specimens. However, it was noted that the quality of the bonds so produced had not been proven satisfactory, and that the arrangement was too complex to be used in the actual production of parts from aluminum. Thus, such diffusion bonding of the so-treated aluminum specimens was never apparently successfully tested and it was concluded that the process would not be suitable for production of diffusion bonded aluminum parts. Further, such sputtering procedure was used only for ultrasonic test standards for flaw definition and was not utilized for diffusion bonding of aluminum parts in preselected areas only, and particularly in combination with superplastic forming.
It is an object of the present invention to provide an improved procedure for producing SPF/DB structures.
Another object of the invention is the provision of a superplastic forming and diffusion bonding procedure particularly for aluminum or aluminum alloy blanks or sheets which provide for uniform rapid removal of surface oxides from the metal blanks to permit effective diffusion bonding of the metal blanks.
A still further object is the provision in the above noted superplastic forming and diffusion bonding process of improved procedure for effective diffusion bonding of aluminum alloy metal blanks or sheets by removal of surface oxides from preselected areas to premit diffusion bonding in such areas while utilizing the initial oxide in other preselected locations to prevent diffusion bonding.